(1) Field of the invention
The present invention relates to a photo detector, for example, which is appropriately used as a quantum dot infrared photo detector.
(2) Description of the Related Art
Conventionally, a quantum dot infrared photo detector (QDIP), for example as shown in FIG. 13(B), has a structure in which a quantum dot layer 102 buries InAs quantum dots 100 with an i-type GaAs buried layer 101; a plurality of the quantum dot layers 102 multilayered (stacked) repeatedly are sandwiched between n-GaAs contact layers (electrode layers) 103.
Since the InAs quantum dots 100 are formed through the self-forming method in a molecular beam epitaxial apparatus, they are distributed within a perpendicular plane to the growth direction. Commonly, in order to enhance the infrared detecting efficiency, as a device in which a plurality layers (e.g., tenth layers) of quantum dot layers 102 each of which includes the InAs quantum dots 100 distributed within this plane are multilayered is used.
FIG. 13(A) shows a conduction band edge profile in a quantum dot infrared detector having such structure. The shape of the conduction band edge profile can be described below.
Since no impurity is doped in the region of the multilayered quantum dot layers 102, electrons are supplied from the n-type GaAs contact layer 103.
Since the ground state (the ground level) of the InAs quantum dots 100 is lower than the energy level of the conduction band edge of the i-type GaAs buried layer 101, the electrons supplied from the n-type GaAs contact layer 103 are positioned at the ground state of the InAs quantum dots 100.
When a probability that an electron occupies a certain energy level becomes ½, it is defined that Fermi level Ef is equal to the energy level of the electron. Here, the Fermi level Ef has become equal to the ground state of the InAs quantum dots 100.
When a system is in a thermal balance state, the Fermi level Ef becomes constant in all of the system.
Accordingly, the conduction band edge profile, as shown in FIG. 13(A), results in a shape that the conduction band edge of portions in which the quantum dot layers 102 are multilayered (stacked) is mounded such that the energy level of the conduction band edge which is equal to the Fermi level of the n-type GaAs contact layer 103 and the ground state of the InAs quantum dots 100 may generally become a constant energy level.
However, since, in the quantum dot layers 102 near the n-type GaAs contact layer 103, the electrons from the n-type GaAs contact layer 103 are excessively supplied, the Fermi level reaches the excited state (excited level) of the InAs quantum dots 100, the mound of the conduction band edge becomes small.
Such quantum dot infrared detector is used in a state that a potential difference is applied between the n-type GaAs contact layers 103.
Here, FIG. 14 shows a conduction band edge profile in such a case that a potential difference is applied between the n-type GaAs contact layers 103 with such a structure. In addition here, it is assumed that the electric field is evenly added to the region in which the quantum dot layers 102 are multilayered.
In this quantum dot infrared detector, as shown in FIG. 14, when infrared lights are made incident from outside, the electrons which are positioned at the ground state within the InAs quantum dots 100 buried in the i-type GaAs buried layer 101 are excited to the excited state by the incident infrared lights. Since the potential difference is applied between the n-type GaAs contact layers 103, the excited electrons flow between the n-type GaAs contact layers 103, and become current (referred to as photo electric current or photo current, hereafter). With detecting this photo current, infrared lights can be detected.
In addition, the prior art documents related to the quantum dot infrared detector are as follows: for example, (1) Eui-Tae Kim et al. “Tailoring detection bands of InAs quantum-dot infrared photo detectors using InxGal-xAs strain-relieving quantum wells”, APPLIED PHYSICS LETTERS, VOLUME 79, NUMBER 20, pp. 3341 to 3343, 12 Nov. 2001; and (2) Eui-Tae Kim et al. “High detectivity InAs quantum-dot infrared photo detectors”, APPLIED PHYSICS LETTERS, VOLUME 84, NUMBER 17, pp. 3277 to 3279, 26 Apr. 2004.
However, in the quantum dot infrared detectors described above, as shown in FIG. 14, even though when no infrared light is made incident, the electrons which are positioned at the n-type GaAs contact layer 103 of low potential caused by the potential difference added between the n-type GaAs contact layers 103 flow between the contact layers and become the dark current.
In other words, in the quantum dot infrared detectors described above, as shown in FIG. 14, in the state that the potential difference is applied, although the energy level of the conduction band edge of the i-type GaAs buried layer 101 which is a few layers distance from the n-type GaAs contact layer 103 of low potential becomes the highest; however, the electrons flowing out from the n-type GaAs contact layer 103 of low potential become dark current that exceed the energy level.
By the way, since the performance of infrared detector is generally determined by a ratio of dark current to photo current, it is important for the performance of the infrared detector to reduce dark current to improve its performance.
In order to reduce dark current, it is conceivable to make the energy level of the conduction band edge of the i-type GaAs buried layer 101 that is a few layers distance from the n-type GaAs contact layer 103 of low potential high.
For example, it is conceivable to make the energy level of the conduction band edge of the buried layer high, by using the AlGaAs having the larger band gap than the i-type GaAs as the material of the buried layer instead of the i-type GaAs.
However, when causing the InAs quantum dot layers 100 to grow, the element temperature is required to rise about 500° C. Even if the AlGaAs layers which are normally grown with about 600° C. element temperature are grown near the quantum dots, it is difficult for the AlGaAs layers to cause it to grow in optimizing the growth condition.